The present invention generally relates to power system protection, and more specifically, to an apparatus and method for identifying a faulted phase in a shunt capacitor bank. The invention also relates to an apparatus and method for identifying the location of the fault (e.g., the section of the bank) in a double ungrounded shunt capacitor bank or double WYE shunt capacitor bank.
Electric utility systems or power systems are designed to generate, transmit and distribute electrical energy to loads. In order to accomplish this, power systems generally include a variety of power system elements such as electrical generators, electrical motors, power transformers, power transmission lines, buses and capacitors, to name a few. As a result, power systems must also include protective devices and procedures to protect the power system elements from abnormal conditions such as electrical short circuits, overloads, frequency excursions, voltage fluctuations, and the like.
Such protective devices and procedures act to isolate some power system element(s) from the remainder of the power system upon detection of the abnormal condition or a fault in, or related to, the power system element(s).
Power system protection may be grouped into six types including: (1) generators and generator-transformer elements (2) transformers, (3) buses, (4) lines (transmission, sub-transmission and distribution), (5) utilization equipment (motors, static loads), and (6) capacitor or reactor banks. As a result, a variety of protective devices are required. Such protective devices may include different types of protective relays, surge protectors, arc gaps and associated circuit breakers and reclosers.
Shunt capacitor banks generally provide capacitive reactive power support/compensation in high or low voltage power transmission systems. Shunt capacitor banks generally comprise a plurality of series/parallel connected capacitor units. Within each capacitor unit are groups of series/parallel-connected capacitors, called capacitor elements. Capacitor elements can be either fused (internally fused) or not fused. Shunt capacitor banks may be either fused or not fused. Fused shunt capacitor banks may be either externally or internally fused (fuses within each capacitor unit).
For externally fused shunt capacitor banks, fuses are externally mounted between at least one of the capacitor units and a capacitor bank fuse bus. If one of the capacitors in a capacitor unit fails, there is an increased voltage and current across the other capacitors in the capacitor unit. Multiple failures in a plurality of capacitors in the capacitor unit cause the fuse to blow, interrupting the fault current. The blown fuse also provides visual identification of the faulted capacitor unit. There is no indication of failed capacitor element(s).
For internally fused shunt capacitor banks, fuses are connected to each of the capacitor elements inside of the capacitor unit. If one of the capacitor elements fails, the fuse interrupts the excessive current and isolates the failed capacitor element. When the failed capacitor element is isolated, there is an increased voltage and current across the other capacitors connected in parallel in the same group in the capacitor unit. There is no visual indication of the failed capacitor element(s) or unit(s).
For fuseless shunt capacitor banks, capacitor units are connected in series strings between phase and neutral. Fuseless shunt capacitor banks are generally applied to systems having voltages of greater than about 34.5 kV. Failure of a capacitor in a capacitor unit shorts a group associated with the failed capacitor. When the short isolates the capacitor group in the capacitor unit, there is an increased voltage and current across the other capacitors in the capacitor unit. There is no visual indication of the failed capacitor element(s) or unit(s).
For unfused shunt capacitor banks, capacitor units are connected using a series/parallel connection of the capacitor units. Unfused shunt capacitor banks are generally applied to systems having voltages of less than about 34.5 kV. There is no visual indication of the failed capacitor element(s) or unit(s).
It is desired that the maintenance time for shunt capacitor banks be as short as possible. Accordingly, it is preferred that a fault in a capacitor unit be identified as soon as possible. Protective devices may be used to identify faults in capacitor units. For shunt capacitor bank applications, a protective device must not only detect the presence of a fault, but must also determine which of the three power system phases is faulted.
When the shunt capacitor bank is grounded, the faulted phase is found by determining whether there is a voltage differential across each phase. However, determining the faulted phase in an ungrounded shunt capacitor bank is more difficult. Accordingly, it is an object of the present invention to provide a protective device which determines a failure in a capacitor unit by identifying the faulted phase and section in an ungrounded shunt capacitor bank.
These and other desired benefits of the preferred embodiments, including combinations of features thereof, of the invention will become apparent from the following description. It will be understood, however, that a process or arrangement could still appropriate the claimed invention without accomplishing each and every one of these desired benefits, including those gleaned from the following description. The appended claims, not these desired benefits, define the subject matter of the invention. Any and all benefits are derived from the multiple embodiments of the invention, not necessarily the invention in general.